His first experience of Australian rules was in the Paul Kelly Cup in 1998 in primary school playing for the junior Pennant Hills Demons AFC. He went on to play a key part in premierships with the junior Demons in 1999 and 2000. He played both rugby league (for Balmain's juniors) and Australian rules until 2000 before breaking the news to his father that he wanted to stop playing league to focus on Australian Rules. He was identified by AFL talent scouts and taken by Sydney in the 2005 Rookie Draft from the NSW/ACT Rams Under 18 team.
Before even making his debut, Kieren made news in 2007 as one of four players allegedly involved in an AFL betting scandal, though many criticised the AFL's decision to identify him as his was only a $5 bet. He was later promoted from the rookie list. Notable for his tackling and running abilities, he plays as a midfielder for the Swans. In round four, 2008, he gained a NAB AFL Rising Star nomination for his role on West Coast Eagles onballer Daniel Kerr in a match the Swans won by 62 points (their biggest win over the Eagles since 2000). In the same season, he also shut down the likes of Shaun Burgoyne and Luke Power in games the Swans have won by considerable margins.
In 2009, he featured in the official advertisement for the AFL, contesting the Australian football against Leon Davis along the floor of on an ice hockey rink. In 2010, Jack continued to improve the offensive side of his game, while able to negate some of the star midfielders of the AFL. His career year was capped off with taking home the Bob Skilton Medal as the Swans' best and fairest player over Shane Mumford and Josh Kennedy. He was also named on the International Rules squad to tour Ireland in October. After a breakthrough season in 2010, Jack struggle through much of 2011 with an ankle injury. He missed 6 games due to injury and was for the most part unable to recapture his form and impact from 2010. He played his 100th AFL game for the Sydney Swans against Essendon in Round 11, 2012. He finished 4th in the 2012 Bob Skilton Medal a week after kicking 2 crucial goals in the Swans' Grand Final victory at the MCG over the Hawthorn Hawks. In 2013, Jack was named co-captain of the Sydney Swans, alongside 2012 Premiership Captain, Jarrad McVeigh. This decision was made after previous co-captain Adam Goodes announced he was stepping down from the position. Jack captained his first match against Gold Coast Suns in Round 3 of the 2013 NAB pre-season cup.